Stenotic lesions may comprise a hard, calcified substance and/or a softer thrombus material, each of which forms on the lumen walls of a blood vessel and restricts blood flow there through. Intra-luminal treatments, such as balloon angioplasty, stent deployment, atherectomy, and thrombectomy are well known and have proven effective in the treatment of such stenotic lesions. These treatments often involve the insertion of a therapy catheter into a patient's vasculature, which may be torturous and may have numerous stenoses of varying degrees throughout its length. In order to place the distal, treatment portion of a catheter within the treatment site, a steerable guidewire is typically introduced and tracked from an incision, through the vessels, and across the lesion. Then, a catheter, e.g., a balloon catheter, perhaps carrying a stent, can be tracked over the guidewire to the treatment site. Ordinarily, the distal end of the guidewire is quite flexible so that as it is directed, or steered through the lumen, it can find its way through the turns of the typically irregular passageway without perforating or otherwise damaging the vessel wall.
In some instances, the extent of occlusion of the lumen is so severe that the lumen is completely or nearly completely obstructed, leaving virtually no passageway for the guidewire. Such a condition may be described as a total occlusion. If this occlusion persists for a long period of time, the lesion is referred to as a chronic total occlusion or CTO. Furthermore, in the case of diseased blood vessels, the lining of the vessels may be characterized by the prevalence of atheromatous plaque, which may form total occlusions. The extensive plaque formation of a chronic total occlusion typically has a fibrous cap surrounding softer plaque material. This fibrous cap may present a surface that is difficult to penetrate with a conventional guidewire, and the typically flexible distal tip of the guidewire may be unable to cross the lesion.
Thus, for treatment of total occlusions, guidewire having stiffer distal tips have been employed to recanalize a total occlusion. However, blood vessels are not straight and fluoroscopic visualization of the natural path through an occlusion is poor because there is little or no flow of radiographic contrast through the occlusion. Therefore, simply using a stiffer guidewire to push through an occlusion increases the risk that the guidewire tip will penetrate the vessel wall.
Atherectomy is another established treatment for occlusions. Atherectomy procedures typically involve inserting a cutting or ablating device through the access artery, e.g. the femoral artery or the radial artery, and advancing it through the vascular system to the occluded region, and rotating the device at high speed via a drive shaft to cut through or ablate the plaque over the wire. The removed plaque or material can then be suctioned out of the vessel or be of such fine diameter that it is cleared by the reticuloendothelial system. Atherectomy devices also present the danger of unwanted perforation of a vessel wall by the material removal device. This can occur when the material removal device improperly engages the vessel wall, for example when the material removal device is not oriented substantially parallel to the axis of the vessel. In this situation, the material removal device, e.g. cutter or abrasive ablator, may improperly engage the vessel wall and cause unwanted damage thereto. Other ablation and discectomy devices also present the danger of damage to a vessel wall.
Thus, there is a need for a device and method to reduce the risk of damage to a vessel wall when a guidewire or a device for performing an atherectomy, discectomy, ablation or similar procedure is crossing an occlusion.
Electrical impedance is the opposition to the flow of an alternating current, which is the vector sum of ohmic resistance plus additional resistance, if any, due to induction, to capacitance, or to both. Bioelectric impedance is known, e.g., for use in measuring body fat composition. For example, bathroom scales may include means to measure body fat composition using bioelectric impedance. According to this technique, a person's body fat is measured by determining the impedance of the person's body to electrical signals, and calculating the percent body fat based upon the measured impedance and other variables, such as height, weight, age, and sex.
Bioelectric impedance is typically determined by supplying a harmless electric current through at least two separated electrodes that contact portions of a body, and measuring a voltage across the body portion. This voltage is measured either (1) via the same electrodes through which current is supplied, or (2) via one or more distinct pairs of voltage-measuring electrodes. The bioelectric impedance is then readily calculated from the current and the measured voltage. The calculated bioelectric impedance may be compared to an expected value or range of common or known values, or it may be compared to one or more bioelectric impedance values previously measured in, and calculated for the same patient.